1. Field of the Invention
The present invention relates to a diesel engine in which premix combustion is implemented, and more particularly to a diesel engine with optimized fuel ignition timing in the premix combustion.
2. Description of the Related Art
In diesel engines, fuel is typically injected close to a compression top dead center of the piston when the temperature and pressure inside the cylinder are high. The injected fuel is mixed with the intake air, forming a mixture, this mixture is ignited producing a flame, and the combustion is maintained by supplying subsequently injected fuel into this flame. In other words, ignition starts during fuel injection.
By contrast, a combustion system called “premix combustion” in which the fuel injection timing is set earlier than the compression top dead center has recently been suggested (for example, see Japanese Patent Applications Laid-open Nos. 2001-20784 and 2002-327638). In the premix combustion, the combustion is started due to initiation of thermal decomposition or oxidation reaction of the fuel accompanying the increase in temperature inside the cylinder in the course of the compression cycle after the end of fuel injection. In other words, the compression self-ignition proceeds once a certain interval elapses after the end of fuel injection. As a result, the mixture is sufficiently leaned and homogenized. Therefore, local combustion temperature decreases and the amount of released NOx(nitrogen oxide) is decreased. Furthermore, the emission of smoke is also inhibited because local combustion is in an air shortage state.
The premix combustion was thus, effective for improving the exhaust gas, but the problem associated therewith was that the fuel ignition timing was difficult to control. This can be explained as follows. In the conventional combustion systems in which ignition starts in the course of fuel injection, the ignition timing can be controlled by controlling the fuel injection timing, but in the premix combustion there is a premixing interval (interval from the end of fuel injection to ignition). Therefore, the ignition timing cannot be fully and stringently controlled by controlling the fuel injection-timing.
In the case of inadequate ignition timing of the fuel (mixture) in the premix combustion, not only the exhaust gas purification effect cannot be obtained, by also the efficiency degradation (fuel consumption deterioration) or combustion noise are generated. For example, when the fuel ignition timing is too early (before the compression top dead center), heat loss is increased and compression with the piston is received after the ignition has been started. As a result, the temperature inside the cylinder increases and there is a risk of NOx generation.
Therefore, it is important to control adequately the ignition timing of fuel during premix combustion.
Japanese Patent Application Laid-open 2002-327638 described a diesel engine comprising a variable supercharger with an adjustable boot pressure and an EGR unit, wherein the target intake air quantity necessary to ignite the fuel at the optimum timing is set for each engine operation state, and the boost pressure and EGR ratio are adjusted so that the actual intake air quantity becomes the target intake air quantity.
However, when the boost pressure was changed to control the intake air quantity, there was a risk of degrading the fuel consumption or producing a negative effect on boost pressure rise during acceleration when the boost pressure failed to correspond to the engine operation state.
Furthermore, because the EGR (exhaust gas recirculation) ratio and intake air quantity change if the boost pressure is changed, it is necessary to control the boost pressure in cooperation with the EGR ratio. Thus, there was a problem that it is difficult to control the intake air quantity. In other words, the intake air quantity is not easy to control to a target value because changes in the intake air quantity caused by changes in the boost pressure and changes in the intake air quantity caused by changes in the EGR ratio are intertwined in a complex manner.